Write an e2e test that generates only valid operations

[abukosek]

The current txgen e2e test generates all kinds of transactions, including invalid ones, so errors are expected and we can't fail the test on error.

We should write a separate test that also uses txgen to generate transactions for the simple-keyvalue runtime, but in a more intelligent way -- the generated insert/get/remove operations should always succeed, so that we can detect when an actual error happens and fail the test.